Fermat prime

A prime number of the form 2^2^n + 1. Any
prime number of the form 2^n+1 must be a Fermat prime.
Fermat conjectured in a letter to someone or other that all
numbers 2^2^n+1 are prime, having noticed that this is true
for n=0,1,2,3,4.

Euler proved that 641 is a factor of 2^2^5+1. Of course
nowadays we would just ask a computer, but at the time it was
an impressive achievement (and his proof is very elegant).

No further Fermat primes are known; several have been
factorised, and several more have been proved composite
without finding explicit factorisations.

Gauss proved that a regular N-sided polygon can be
constructed with ruler and compasses if and only if N is a
power of 2 times a product of distinct Fermat primes.